LONDON (AP) -- Tense post-Brexit relations between Britain and the European Union face further strain on Wednesday, when the U.K. calls for major changes to trade rules agreed on by both sides.
Brexit minister David Frost will set out proposals for smoothing out trade arrangements for Northern Ireland, the only part of the U.K. that has a land border with the 27-nation bloc. The divorce deal agreed before Britain's departure means customs and border checks must be conducted on some goods moving between Northern Ireland and the rest of the U.K.
The regulations are intended to keep an open border between Northern Ireland and EU member Ireland, a key pillar of Northern Ireland's peace process. But they have angered Northern Ireland's British unionists, who say they amount to a border in the Irish Sea and weaken ties with the rest of the U.K.
Britain accuses the EU of taking a "purist" approach to the rules that is causing unnecessary red tape for businesses, and has called for the bloc to show "pragmatism."
The U.K. has threatened to unilaterally suspend parts of the agreement if changes aren't made, in what would be a major escalation of the dispute, though it isn't expected to take that step this week.
Frost said Monday that the agreement "isn't sustainable in the way it's working at the moment."
"All options are on the table," he said.
The EU says Prime Minister Boris Johnson's government knew full well that there would be checks when it signed the Brexit deal.
"Britain decided itself to leave the single market of the European Union, to apply trade rules, to apply red tape to its goods that are leaving Britain, to goods that are coming into Britain," Irish European affairs minister Thomas Byrne said.
Last month, the two sides gave themselves breathing time by delaying until the end of September a ban on chilled meats such as sausages from England, Scotland and Wales from going to Northern Ireland.
The "sausage war" has been the highest-profile element of the U.K.-EU dispute, raising fears that Northern Ireland supermarkets may not be able to sell British sausages, a breakfast staple.
Archie Norman, chairman of food and fashion chain Marks and Spencer, said that the new rules meant there would be "gaps on the shelves" in Northern Ireland at Christmas.
"This Christmas, I can tell you already, we're having to make decisions to delist product for Northern Ireland because it's simply not worth the risk of trying to get it through," he told the BBC.